1. Field of the Invention
The present invention relates to a technology for removing dust and other foreign substances adhered to a focal plane shutter in an optical device such as a digital camera.
2. Description of the Related Art
In the past, the presence of dust in the vicinity of the focal plane of a shooting lens used in a lens-exchangeable type of digital single-lens reflex camera created a problem because the dust projected its shadow onto a solid-state image sensing device. Such dust is believed to find its way from the outside when lenses are exchanged, or to be produced as fine abrasive dust consisting of resin and other structural member materials as a result of movement of the shutter and mirrors in the camera. If such dust entered the space between the cover glass used for the protection of the solid-state image sensing device and the optical filters, such as an infrared cutoff filter or a low-pass optical filter, etc., which were arranged in front of the cover glass, the camera had to be taken apart in order to remove the dust. Accordingly, methods used to address the problem consisted in creating a sealed structure adapted to prevent dust from entering the space between the optical filters and the cover glass of the solid-state image sensing device.
However, the problem was that the dust adhered to the surface of the side of the optical filter opposite the solid-state image sensing device and, when it was in the vicinity of the focal plane, the dust created a shadow that still ended up being projected onto the solid-state image sensing device. The mechanism, whereby dust adheres to the surface of the side (lens unit side) of the optical filter opposite the solid-state image sensing device is believed to be as follows.
A focal plane shutter is usually arranged in extended condition (in which the shutter is closed) in front of the optical filter. Therefore, a structure is created, in which dust generated for the above-described reasons does not directly adhere to the optical filter. Namely, it is believed that, after adhering to the extended shutter blades, the dust generated for the above-mentioned reasons is scattered by the opening/closing operation of the shutter and adheres to the optical filter by passing through the shutter when it is opened.
In view of this problem, a double light-shielding system shutter, which is constructed with a view to impede the adhesion of dust to the optical filter, was proposed in Japanese Patent Application No. 08-211445. It has shutter blades reduced in weight in order to raise the travel speed of the shutter blades and is adapted to increase shielding with the help of two groups of blades maintained in extended condition while in the shooting-ready state in order to avoid shielding-related problems caused by the thinning of the shutter blades. In accordance with this construction, even if the dust is scattered as a result of a shutter release operation first actuating the shutter blade group that has dust adhered thereto, another shutter blade group on the optical filter side is in an extended condition. Therefore, even if the scattered dust moves towards the optical filter, the latter is shielded by the shutter blade group maintained in the extended condition and the dust cannot reach the optical filter, which prevents the dust from adhering to the optical filter. Moreover, since the shutter blade group on the optical filter side, which had little dust adhesion, is actuated after actuating the first shutter blade group, the construction makes it possible to greatly reduce the amount of dust adhered to the optical filter in comparison with the focal plane shutter of the above-mentioned conventional construction.
In addition, Japanese Patent No. 3576703 disclosed shutter overloading as an operation performed in an optical device in order to increase shutter speed.
In the double light-shielding system shutter described in Japanese Patent Application No. 08-211445, dust adhesion to the optical filter cannot be completely prevented if the operation of the other shutter blade group starts before dust scattered by the initially actuated shutter blade group completely settles. Moreover, the need for special operations to ensure that the two groups of blades are in extended condition for the purpose of shielding in the shooting-ready state has led to problems due to the increased time lag between pressing the release button and the start of shooting.